1. Field
Embodiments of the disclosure relate generally to the field of bonding of transparent substrates and more particularly to a method for room temperature laser bonding of a first laser wavelength transparent substrate to a second substrate with an intermediate heat absorption layer.
2. Background
Bonding of glass-to-glass substrates and other combinations of transparent and non-transparent substrates for biological slides and microfluidics applications as well as other applications typically requires heating of the substrates to obtain bonding diffusion of the materials across the substrate boundaries unless adhesives are employed. Various examples of current bonding practices are fusion bonding, anodic bonding of sodium rich glass to semiconductors and adhesive bonding.
Fusion bonding glass-to-glass is effective on polished, or low roughness glass surfaces. To achieve a strong, bubble free bond, typically the surface finish should be on the order of a few Angstrom RA. The process generally involves placing the two glass substrates in contact with each other and then applying pressure and heat. The pressure can range from the weight of the upper glass substrate to a load place on top of the glass. Special material must be used to prevent the weight from sticking to the glass. The bulk substrate is usually brought up to at least the first transition temperature (softening temperature) of the glass. For all practical purposes, the glass surfaces melt together and become one. This process is not very robust against environmental particles that are commonly found in a clean room environment. A 50 nm diameter particle, for example, will cause the glass not to bond in that particular area and cause a glass bubble which is apparent by the presence of Newton Rings.
This process can be assisted by treating the surface with ions such as calcium and activating the surface with Hydrofluoric Acid. Such treatments tend to lower the bonding temperatures but aggravate the contamination problem. Contamination becomes more difficult because the particulate does not have the ability to deform the glass such that the particle of contamination will recess out of the way and not hold the two surfaces apart.
Fusion bonding has two competing issues that cause a low yield; the glass surface must be absolutely clean in order to not create air bubbles at low temperatures, and when higher temperatures are used, while air gaps become less a problem, the surface of the glass becomes distorted and must be reprocessed in order to make it optically clear again. Higher temperatures can also cause the glass to become hazed or yellowish.
While there are a few exceptions, it is generally not possible to bond glass-to-glass with an Anodic bonding process. This process is usually reserved for bonding glass to silicon. Anodic bonding is usually performed using glass substrate with sodium as one of its constituents. The temperature is generally elevated to approximately 400 degrees Celsius. A potential difference is then applied to drive the sodium atoms across the boundary of the glass-silicon assembly. This process creates a sodium-oxide bond across the boundary. This process usually leaves the surface of the glass transparent and smooth. However, it is assumed that the bonding process is taking place near a channel, the depletion of the sodium atoms from the surface of the glass near the bonded interface layer, leave the glass sodium rich. This surface is then positively charged. Such a charge on the surface of the glass can easily interfere with downstream processes during the use of the chip.
There are adhesives specifically designed to bond glass to glass. While adhesive is easy to apply, it is very hard to make a bubble free joint. It is also very hard to pattern adhesive such that the bond line is complete but does not squeeze out from between the surfaces being bonded and into a neighboring channel. Adhesives can be hazardous to the downstream process. Certain adhesive compositions can kill the biology that the component is being made to house.
Each of the above bonding processes does not render a chemically inert bonding process. In each case the bond lines are not robust against strong concentrations of acid or bases. They will tend to etch at a much higher rate than that of the bulk surface. The higher etch rate can cause small crevasse that are hard to clean or harm the flow of liquid in the channel assembly in the case of micro-fluidics.
Because each of the above typically require heat, it is necessary to match the thermal-coefficient-of-expansion of each material. If this is not done, when the material returns to room temperature the bonded component will warp and/or break. The adhesive joint will fail in shear or peel if the use temperature is different from the bonding temperature; adhesive shear strength is usually low.
It is therefore desirable to provide a glass-to-glass or other substrate bonding process providing bonding times in a range of minutes as opposed to hours for anodic bonding or heat diffusion bonding. It is further desirable to provide a bonding process with a tolerance to dirt, which can bond through 100 nm diameter particles. It is also desirable that the bonding process provide a selectable width bond-line width 10 to 100 μm with bonded un-bonded discrimination of 1 μm. Additionally, it is desirable that the bonding process is inert and does not aver etch in HF/Sulfuric/KOH (as with diffusion bonding) and does not change the charge on the surface of the glass as with anodic bonding. It is also desirable that the bond-line is virtually transparent and the bonding process can structure the bond line as well as conductors and non-conductors within the bonded structure on the same surface. Finally, it is desirable that bonding can be accomplished on a fluidic device loaded with live cultures such as yeast, anthrax or other biological materials without harming them.